The present invention relates to lasers and more particularly to lasers employing phase conjugate mirrors for correcting wavefront distortions. The present invention further relates to an apparatus and method for employing Stimulated Brillouin scattering media for creating matching or compensating sets of moving reflective gratings.
In an ideal laser, the resonant cavity is perfectly tuned and could support a single oscillation mode containing all the stored energy. The lasing medium can efficiently impart energy to the oscillating radiation but would not otherwise interact with the radiation. However, an ideal laser does not exist and many problems exist that tend to degrade the operation of lasers and laser systems, especially those utilizing advanced technology lasing media or optical elements.
The optical elements defining a laser resonant cavity are subject to several disturbing effects including vibration, misalignment, or thermal stress. Advanced lasers or laser systems may employ many specialized elements such as steering mirrors, Q-switches, lenses, or beam clippers. All of these elements must be precisely aligned and relatively fixed with respect to each other in order to support a single mode within the laser gain media linewidth. Vibration or poor construction technique can lead to misalignment or random re-positioning. In addition, the high energy density found in high energy state of the art lasers can cause thermal distortion of reflective surfaces altering reflectivity and alignment. These effects alter the laser resonant cavity dimensions which degrade cavity tuning, resulting in poor mode selection or rejection.
At the same time, the lasing medium itself, unlike an idealized medium, disturbs the radiation by virtue of aberrations. These aberrations include media non-uniformities such as defects or impurities in solid state media, thermally induced index changes, or turbulence and concentration variations in gas or liquid media. Aberrations in the idealized path or transfer function of laser radiation causes optical path variations, which results in decreased spatial intensity at the focal plane as a consequence of defocussing and distortion of wavefronts. The multipass nature of laser oscillators greatly enhances any distortions due to aberrations multiple encounters of the aberrations by the wavefronts. The result is an oscillation with greatly distorted modes which degrades the ability to efficiently extract from the inverted population of the cavity. In addition, time varying media aberrations can cause wavelength or frequency variations in the radiation injected into the cavity by the media.
These undesirable conditions result in lower system efficiency, and keep a laser from performing at its diffraction limit, ie., optimum focusing and energy transfer capability thus reducing the extractable power. They also lead to spectral broadening of the laser output which contains distorted wavefronts. In order to prevent these undesirable results, several techniques have been proposed for many laser applications.
Previously, in order to solve these problems, a high degree of accuracy in the fabrication of the optical components (typically better than 1/10 wavelength) was required, and oscillator cavities that were extremely mechanically stable were utilized. Accurate alignment of focusing elements, such as cavity mirrors, apertures and the like are critical in the conventional laser resonator. This accuracy is achieved only at great expense both in terms of cost and fabrication complexity. In addition, the requirement for extreme physical accuracy and rigidity severely limits the use of lasers outside of a laboratory setting. Most laser components are generally subjected to a variety of forces requiring structural reinforcement to maintain optical accuracy. Therefore, it is desirable to reduce the alignment sensitivity of individual optical elements.
Two approaches known for minimizing alignment sensitivity involve the use of retroreflectors such as "corner cube" or "cats-eye" reflectors. Such reflective elements reflect collimated radiation in an angular direction identical to the incident angle, which thereby eliminates the necessity of precise angular alignment of the reflecting element surface. The corner cube, however, creates polarization distortions which make it unsuitable for many applications and the cat's eye reflector suffers from power limitations caused by a small spot size.
An approach proposed for addressing extra-cavity aberrations such as atmospheric perturbations is the application of a wavefront correction device in the laser optical element chain. Two examples of a proposed type of wavefront correction device are found in the mechanically deformable mirrors described in U.S. Pat. Nos. 3,731,103 and 4,005,935 issued to the present assignee of the instant application. While these systems perform their designed functions, they require external laser beam sampling to provide a feedback loop for mirror deformation control which increases complexity and expense. Deformable mirror systems also suffer from restricted spatial bandwidth, speed of response, the requirement of high voltage power supplies, and mirror pin cushion distortions and print through. In addition, such systems have a high degree of complexity for more advanced applications which increases system cost and lowers reliability.
A more recent development in the field of lasers is the use of phase conjugation to prevent inter-cavity aberrations and operational irregularities from degrading laser operation. Such lasers employ one or more phase conjugate reflectors, realized by nonlinear optical interactions which are used to generate the spatial phase conjugate of a distorted wavefront, which when retransmitted through the original distorting optical path forms a corrected beam. Therefore, planar waves initially injected into a laser cavity by a lasing medium or otherwise reflected from a surface into a laser gas medium are returned as plane waves. In phase conjugate resonators a planar or other suitably chosen or well figured mirror may substitute for the planar wave injected into the laser cavity.
The nonlinear interactions of a phase conjugate reflector automatically perform a phase front correction without external wavefront sensing or electronic controls. This can provide high optical speed and economy of design.
Nonlinear phase conjugation offers advantages over conventional means of aberration control in that it does not require electronic feedback, auxiliary power supplies, mirror cooling or many other features. The use of phase conjugate mirrors has greatly improved the far field patterns from lasers by decreasing the effects of internal or external aberrations. Also efficiency of energy extraction can be increased by recovering radiation that is otherwise lost by diffraction in conventional resonators. This is not accomplished, however, without additional problems.
Because of its high reflectivity and efficiency, the most promising phase conjugate medium is one that operates by using the Stimulated Brillouin Scattering (SBS) process. SBS phase conjugate reflectors function by establishing a grating which reflects incident radiation. The grating is in fact created by the interaction of incident radiation with the medium comprising the phase conjugate reflector. The grating is an acoustic wave which moves at sonic speed through the phase conjugate reflector medium in the same direction of travel as the incident radiation. Therefore, incident radiation reflected from the moving grating undergoes a small doppler shift. Even though the wavelength shift is small, typically a few gigahertz, it is also cumulative due to the multipass nature of the resonant cavity. Whenever radiation traverses or passes through the laser resonant cavity it undergoes an additional wavelength shift. This results in a wavelength or frequency "walk-off". The term "walk-off" is used to denote the fact that pass by pass the radiation walks away from the peak gain wavelength of the lasing medium.
The shift in frequency or wavelength for reflected radiation leads to several problems. First, it can produce a series of output frequencies which creates undesirable spectral broadening. Second, the detuning of the radiation wavelength, away from the wavelength of maximum laser gain for radiation oscillating in the laser resonant cavity leads to decreased efficiency for energy transfer and lower output power. Third, at some point the "walk-off" can cause the radiation to reach an unstable point for supporting the phase conjugation process as where Stimulated Brillouin Scattering cannot be achieved due to a low intensity function. Fourth, while for many wide bandwidth laser gain media, limited "walk-off" can be tolerated, eventually the radiation is shifted outside of the laser gain linewidth such that steady-state operation is not possible.
Therefore, what is needed is a method for preventing the frequency walk-off in phase conjugate reflectors for use in high quality or high power laser sources. One solution would be to use an SBS mirror with a rapid flowing gas cell. The gas would flow at sonic velocity in a directive opposite to the incident laser beam.
In order to solve some of the above problems, the present invention provides a method and apparatus for achieving phase conjugate reflection in a laser apparatus without walk-off. It is also a purpose of the present invention to achieve phase conjugation in a laser without reference to external compensation schemes.
Another purpose of this invention is to provide an improved laser that corrects for aberrations in laser wavefronts to allow them to operate at or near the diffraction limit.